drm/vc4: Report HVS underrun errors

Add a debugfs entry and helper for reporting HVS underrun errors as
well as helpers for masking and unmasking the underrun interrupts.
Add an IRQ handler and initial IRQ configuration.
Rework related register definitions to take the channel number.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220155124.25022-2-paul.kocialkowski@bootlin.com